1. Technical Field of the Invention
The present invention relates to an information reproduction apparatus and an information reproduction method for reading out and reproducing information material having a plurality of clips.
2. Description of the Related Art
In recent years, the techniques of compression-coding image information and audio information with minimum deterioration and then decoding the compressed-coded information material with a reproducing means have been developed. For example, the MPEG (Motion Picture Experts Group) system standard has been proposed as a scheme for compression-coding and then decoding (expanding) image information and audio information. An audio-video material or the like that has been compressed and multiplexed according to the MPEG system standard consists of a plurality of clips as shown in FIG. 10. Each clip consists of a plurality of clusters, four clusters in the example of FIG. 10. A "cluster" is a unit that is an integral multiple of a unit (for instance, 512 bytes) that is formed on a hard disk and generally called a sector. The example of FIG. 10 shows two clips, i.e., a certain clip CP2 and a preceding clip CP1.
In the conventional example of FIG. 10, information material (MPEG stream) 2 having clips CP1, CP2, etc. is recorded on a hard disk (HDD) 1. The information material 2 recorded on the hard disk 1 is to be decoded by a decoder. However, direct access to the information material 2 cannot be effected between the hard disk 1 and the decoder 2. This is because the MPEG stream is recorded on the hard disk 1 so as to be divided into certain units (for example, 4-byte units; hereinafter referred to as cells 1a) and a DMA buffer 3 needs to be used as a buffer for absorbing a size difference between the cell 1a of the hard disk 1 and the cluster CT. That is, in accessing the hard disk 1, the information material 2 needs to be temporarily stored in a buffer 3A or a buffer 3B.
To decode the clusters of the clips CP1 and CP2, for instance, by sending them to decoders 4a and 4b consecutively, a switcher 5 is used as shown in FIG. 10. The first cluster CT1 of the preceding CP1 is stored in the buffer 3A and reproduced by a decoder 4a, and the next cluster CT2 is stored in the buffer 3B and decoded by the decoder 4b. The switcher 5 sequentially outputs the clusters CT as a video signal, an audio signal, or the like by switching between the decoders 4a and 4b. As the switcher 5 switches between the decoders 4a and 4b, the clusters CT of the preceding clip CP1 and the clusters CT of the (ensuing) clip CP2 are reproduced consecutively.
When a user reproduces and uses audio-video material with a VTR or the like, he may want to start reproduction of a clip of the audio-video material from a certain position thereof as well as from its beginning. For this purpose, equipment such as a VTR is generally provided with such functions as FF (fast forward) and REW (fast reverse).
With equipment such as a VTR, when a user uses a system for decoding (reproducing) an audio-video material that has been coded according to the MPEG system scheme, he may want to start reproduction of a clip of a decoded audio-video material from a certain position thereof This operation can be realized by the following two methods:
Method A: Encoding a clip so that certain codes to serve as "markers" are inserted everywhere in the clip, and supporting a function that enables reading a clip of data to be started from a desired marker position in reproducing the clip.
Method B: Calculating a reproduction start position based on the compression rate of a clip and a desired time point (position). According to this scheme, for example, if the compression rate is 10 Mbps and it is desired to start reproduction from a time point 120 seconds after the present time, reproduction may be started from a position 1,200 Mbits downstream of the present position.
The above-described method A is employed in the DVD (digital versatile disc) etc. Although this method has the advantage that reproduction can be started precisely from a desired position, a function for handling "markers" is necessary, resulting in the disadvantage that the system configuration becomes more complex.
On the other hand, method B has the advantage that the system remains simple because it can be implemented easily, though it has the disadvantage that reproduction can be started only from an approximate position. However, there are a considerable number of uses in which reproduction need not always be started from a precisely determined position, that is, an accuracy of about several seconds is allowable. An example of such uses is the case that a user wants to reproduce a clip again from a somewhat upstream position. Method B is most appropriate for such common uses.
However, in method B, if reproduction is started from an arbitrary position, that is, if no limitation is imposed on the reproduction start position, there may occur a case that reproduction (i.e., an operation in which a clip of a coded audio-video material is supplied to a decoder and decoded there) is started from a certain position of a minimum unit called a "packet" that is prescribed by the MPEG2 system standard. In this case, the decoder may not be able to process the data correctly.
An object of the present invention is therefore to solve the above problems of method B and to thereby provide an information reproduction apparatus and an information reproduction method which can perform special reproducing operations such as fast forward and fast reverse on an audio-video material in a precise manner.